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Didelon P.,University Paris Diderot | Motte F.,University Paris Diderot | Tremblin P.,University Paris Diderot | Tremblin P.,University of Exeter | And 43 more authors.
Astronomy and Astrophysics | Year: 2015

Context. The surroundings of H ii regions can have a profound influence on their development, morphology, and evolution. This paper explores the effect of the environment on H ii regions in the MonR2 molecular cloud. Aims. We aim to investigate the density structure of envelopes surrounding H ii regions and to determine their collapse and ionisation expansion ages. The Mon R2 molecular cloud is an ideal target since it hosts an H ii region association, which has been imaged by the Herschel PACS and SPIRE cameras as part of the HOBYS key programme. Methods. Column density and temperature images derived from Herschel data were used together to model the structure of H ii bubbles and their surrounding envelopes. The resulting observational constraints were used to follow the development of the Mon R2 ionised regions with analytical calculations and numerical simulations. Results. The four hot bubbles associated with H ii regions are surrounded by dense, cold, and neutral gas envelopes, which are partly embedded in filaments. The envelope's radial density profiles are reminiscent of those of low-mass protostellar envelopes. The inner parts of envelopes of all four H ii regions could be free-falling because they display shallow density profiles: ρ(r) â r- q with \hbox{$q \leqslant 1.5$}. As for their outer parts, the two compact H ii regions show a ρ(r) â r-2 profile, which is typical of the equilibrium structure of a singular isothermal sphere. In contrast, the central UCH ii region shows a steeper outer profile, ρ(r) â r-2.5, that could be interpreted as material being forced to collapse, where an external agent overwhelms the internal pressure support. Conclusions. The size of the heated bubbles, the spectral type of the irradiating stars, and the mean initial neutral gas density are used to estimate the ionisation expansion time, texp ~ 0.1 Myr, for the dense UCH ii and compact H ii regions and ~ 0.35 Myr for the extended H ii region. Numerical simulations with and without gravity show that the so-called lifetime problem of H ii regions is an artefact of theories that do not take their surrounding neutral envelopes with slowly decreasing density profiles into account. The envelope transition radii between the shallow and steeper density profiles are used to estimate the time elapsed since the formation of the first protostellar embryo, tinf ~ 1 Myr, for the ultra-compact, 1.5-3 Myr for the compact, and greater than ~6 Myr for the extended H ii regions. These results suggest that the time needed to form a OB-star embryo and to start ionising the cloud, plus the quenching time due to the large gravitational potential amplified by further in-falling material, dominates the ionisation expansion time by a large factor. Accurate determination of the quenching time of H ii regions would require additional small-scale observationnal constraints and numerical simulations including 3D geometry effects. © ESO, 2015.

Farrugia C.J.,University of New Hampshire | Mostl C.,Austrian Academy of Sciences | Galvin A.B.,University of New Hampshire | Leitner M.,University of Innsbruck | And 9 more authors.
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2011

We report a comprehensive analysis of in situ observations made by Wind and the STEREO probes (STA, STB) of a complex interaction between a magnetic cloud (MC) and a corotating interaction region (CIR) occurring near the heliospheric current sheet (HCS) on November 19-21, 2007. The probes were separated by 0.7. AU . (~40°) with a spread in heliographic latitudes (4.8,° 2.2,° and -0.4,° for STB, Wind and STA, respectively). We employ data from the MFI, SWE and 3DP instruments on Wind, and the PLASTIC and IMPACT suites on STEREO. STB, located east of Earth, observed a forward shock followed by signatures of a MC. The MC took the role of the HCS in that the polarity of the interplanetary magnetic field (IMF) on exit was the reverse of that on entry. A passage through a plasma sheet was observed. Along the Sun-Earth line Wind observed a stream interface (SI) between a forward and a reverse shock. A MC, compressed by the CIR, was entrained in this. STA, located 20° to the west of Earth, saw a MC which was not preceded by a shock. A SI trailed the transient. The shocks are examined using various methods and from this it is concluded that the forward shock at Wind-but not at STB-was driven by the MC. Examining the MC by Grad-Shafranov reconstruction, we find evidence of a double-flux rope structure at Wind and STA and possibly also at STB. The orientations are at variance with the notion of a large-scale flux tube being observed at the three spacecraft. We find consistency of this with the directional properties of the solar wind "strahl" electrons. We examine aspects of the geomagnetic response and find a double-dip storm corresponding to the two interplanetary triggers. The minimum Dst phase was prolonged and the geoeffects were intensified due to the interaction. We conclude that while the formation of compound streams is a common feature of interplanetary space, understanding their components when CIRs are involved is a complicated matter needing numerical simulations and/or more in situ observations for its complete elucidation. © 2010 Elsevier Ltd.

Perez-De-Tejada H.,National Autonomous University of Mexico | Lundin R.,Swedish Institute of Space Physics | Durand-Manterola H.,National Autonomous University of Mexico | Barabash S.,Swedish Institute of Space Physics | And 3 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

Measurements conducted with the ASPERA-4 instrument and the magnetometer of the Venus Express spacecraft show that the kinetic pressure of planetary O + ion fluxes measured in the Venus wake can be significantly larger than the local magnetic pressure, and as a result, those ions are not solely being driven by magnetic forces but also by the kinetic energy of the solar wind. Beams of planetary O+ ions with those properties have been detected in several orbits of the Venus Express through the Venus wake as the spacecraft traverses by the noon-midnight plane along its near-polar trajectory. The momentum flux of the O+ ions leads to superalfvenic flow conditions. It is suggested that such O+ ion beams are produced in the vicinity of the magnetic polar regions of the Venus ionosphere where the solar wind erodes the local plasma leading to plasma channels that extend downstream from those regions. The distribution of the number of cases where superalfvenic and subalfvenic conditions are measured along the Venus Express trajectory leads to dominant values when the total kinetic plasma pressure (including that of the solar wind protons) and the magnetic pressure are comparable, thus suggesting a possible equipartition of energy between the plasma and the magnetic field. ©2013. American Geophysical Union. All Rights Reserved.

Kis A.,Geodetic and Geophysical Institute | Kis A.,French National Center for Scientific Research | Agapitov O.,French National Center for Scientific Research | Agapitov O.,Taras Shevchenko National University | And 5 more authors.
Astrophysical Journal | Year: 2013

It is well known that shocks in space plasmas can accelerate particles to high energies. However, many details of the shock acceleration mechanism are still unknown. A critical element of shock acceleration is the injection problem; i.e., the presence of the so called seed particle population that is needed for the acceleration to work efficiently. In our case study, we present for the first time observational evidence of gyroresonant surfing acceleration in front of Earth's quasi-parallel bow shock resulting in the appearance of the long-suspected seed particle population. For our analysis, we use simultaneous multi-spacecraft measurements provided by the Cluster spacecraft ion (CIS), magnetic (FGM), and electric field and wave instrument (EFW) during a time period of large inter-spacecraft separation distance. The spacecraft were moving toward the bow shock and were situated in the foreshock region. The results show that the gyroresonance surfing acceleration takes place as a consequence of interaction between circularly polarized monochromatic (or quasi-monochromatic) transversal electromagnetic plasma waves and short large amplitude magnetic structures (SLAMSs). The magnetic mirror force of the SLAMS provides the resonant conditions for the ions trapped by the waves and results in the acceleration of ions. Since wave packets with circular polarization and different kinds of magnetic structures are very commonly observed in front of Earth's quasi-parallel bow shock, the gyroresonant surfing acceleration proves to be an important particle injection mechanism. We also show that seed ions are accelerated directly from the solar wind ion population. © 2013. The American Astronomical Society. All rights reserved.

Zettergren M.,Embry - Riddle Aeronautical University | Semeter J.,Boston University | Burnett B.,Boston University | Oliver W.,Boston University | And 3 more authors.
Annales Geophysicae | Year: 2010

The work presents a data-model synthesis examining the response of the auroral F-region ion temperature, composition, and density to short time scale (<1 min) electric field disturbances associated with auroral arcs. Ion temperature profiles recorded by the Sondrestrom incoherent scatter radar (ISR) are critically analyzed with the aid of theoretical calculations to infer ion composition variability. The analyses presented include a partial accounting for the effects of neutral winds on frictional heating and show promise as the groundwork for future attempts to address ion temperature-mass ambiguities in short-integration ISR data sets. Results indicate that large NO+ enchancements in the F-region can occur in as little as 20 s in response to impulsive changes in ion frictional heating. Enhancements in molecular ion density result in recombination and a depletion in plasma, which is shown to occur on time scales of several minutes. This depletion process, thus, appears to be of comparable importance to electrodynamic evacuation processes in producing auroral arc-related plasma depletions. Furthermore, the potential of ionospheric composition in regulating the amounts and types of ions supplied to the magnetosphere is outlined.

Bavassano Cattaneo M.B.,Instituto Of Fisica Dello Spazio Interplanetario | Marcucci M.F.,Instituto Of Fisica Dello Spazio Interplanetario | Bogdanova Y.V.,University College London | Reme H.,CESR | And 3 more authors.
Annales Geophysicae | Year: 2010

During a long lasting period of northward interplanetary magnetic field and high solar wind speed (above 700 km/s), the Cluster spacecraft go across a number of very large rolled-up Kelvin-Helmholtz (KH) vortices at the dusk magnetopause, close to the terminator. The peculiarity of the present event is a particular sequence of ions and electrons distribution functions observed repeatedly inside each vortex. In particular, whenever Cluster crosses the current layer inside the vortices, multiple field-aligned ion populations appear, suggesting the occurrence of reconnection. In addition, the ion data display a clear velocity filter effect both at the leading and at the trailing edge of each vortex. This effect is not present in the simultaneous electron data. Unlike other KH studies reported in the literature in which reconnection occurs within the vortices, in the present event the observations are not compatible with local reconnection, but are accounted for by lobe reconnection occurring along an extended X-line at the terminator in the Southern Hemisphere. The reconnected field lines "sink" across the magnetopause and then convect tailward-duskward where they become embedded in the vortices. Another observational evidence is the detected presence of solar wind plasma on the magnetospheric side of the vortices, which confirms unambiguously the occurrence of mass transport across the magnetopause already reported in the literature. The proposed reconnection scenario accounts for all the observational aspects, regarding both the transport process and the kinetic signatures. © 2010 Author(s).

Waara M.,Swedish Institute of Space Physics | Nilsson H.,Swedish Institute of Space Physics | Stenberg G.,Swedish Institute of Space Physics | Andre M.,Swedish Institute of Space Physics | And 2 more authors.
Annales Geophysicae | Year: 2010

We present a case study of significant heating (up to 8 keV) perpendicular to the geomagnetic field of outflowing oxygen ions at high altitude (12 R E) above the polar cap. The shape of the distribution functions indicates that most of the heating occurs locally (within 0.2-0.4 RE in altitude). This is a clear example of local ion energization at much higher altitude than usually reported. In contrast to many events at lower altitudes, it is not likely that the locally observed wave fields can cause the observed ion energization. Also, it is not likely that the ions have drifted from some nearby energization region to the point of observation. This suggests that additional fundamentally different ion energization mechanisms are present at high altitudes. One possibility is that the magnetic moment of the ions is not conserved, resulting in slower outflow velocities and longer time for ion energization. © 2010 Author(s).

Shi J.,Chinese Academy of Sciences | Guo J.,National Center for Space Weather | Dunlop M.,Rutherford Appleton Laboratory | Zhang T.,Austrian Academy of Sciences | And 5 more authors.
Annales Geophysicae | Year: 2012

A data set of the Cluster cusp crossings over a 5-year period is studied for the interhemispheric comparison of the dipole tilt angle effect on the latitude of the mid-altitude cusp. The result shows that the dipole tilt angle has a clear control of the cusp latitudinal location. Although, the northern cusp moves 0.054° ILAT for every 1° increase in the dipole tilt angle at the mean altitude of 5.2RE, the southern cusp moves 0.051° ILAT for every 1° increase in the dipole tilt angle at the mean altitude of 6.6RE. The northern cusp dependence agrees with the trend formed by other observations of different satellites for different altitudes, whereas the southern cusp does not. We therefore suggest that there is an inter-hemispheric difference in the dipole tilt angle dependence of cusp, latitudinal location, which has an impact on other observations of different satellites in different altitudes. © 2012 Author(s).

He Z.H.,CAS Center for Space Science and Applied Research | Liu Z.X.,CAS Center for Space Science and Applied Research | Chen T.,CAS Center for Space Science and Applied Research | Shen C.,CAS Center for Space Science and Applied Research | And 3 more authors.
Annales Geophysicae | Year: 2010

The relationship between the average structure of the inner magnetospheric large-scale electric field and geomagnetic activity levels has been investigated by Double Star TC-1 data for radial distances ρ between 4.5 RE and 12.5 RE and MLT between 18:00 h and 06:00 h from July to October in 2004 and 2005. The sunward component of the electric field decreases monotonically as ρ increases and approaches zero as the distance off the Earth is greater than 10 RE. The dawn-dusk component is always duskward. It decreases at about 6 RE where the ring current is typically observed to be the strongest and shows strong asymmetry with respect to the magnetic local time. Surprisingly, the average electric field obtained from TC-1 for low activity is almost comparable to that observed during moderate activity, which is always duskward at the magnetotail (8 RE∼12 RE). © Author(s) 2010.

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